In Hawaii Ormat Technologies, a geothermal company, have drilled into a magma chamber. The magma swells into the drill hole for a short way and then cools, plugging the hole long before it reaches the surface.

I believe what kind of volcano one was drilling would effect how the magma behaves as well. Hawaiian lava is quite easy flowing and does not contain much dissolved gasses, which is why Hawaiian volcanos are not typically explosive, unlike say Vesuvius, who's lava does contain a large amount of dissolved gas, and why when it blows it does so quite explosively.

So can eruptions be avoided, provided we know where/when they are likely to occur?

I think they can be predicted with GPS technology that monitors undulations in the ground level (caused by the movement of magma).

Drilling a volcano to reduce pressures is analogous to putting a flea on an elephant. Drill holes are simply too small to relieve the pressure in magma chambers. And then there is the worst possible, though highly unlikely, scenario- a drill hole into a gas charged magma could be the straw that breaks the camel's back, causing an explosive eruption (which would have happened in the very near future anyway). The likely result of drilling a magma chamber is a plugged hole as the lava cools.GPS monitoring is one of several techniques used to monitor volcanoes. Others include seismic, radar interferometry, gas sampling, heat flow, vegetation studies, etc. Harmonic tremors suggests that the magma is moving near surface, and is probably the best indicator of imminent eruption.

There was a discussion in the geology section a year or two ago and it details many more reasons why it won't work. I can't find it right now.

Ormat Technologies probably lost a lot of money when they drilled into the magma chamber accidentally. A drill bit that they would be using cost somewhere around $50,000.00 and the pipe they would have used would have been even more expensive. The bottom 4-12 joints of drill pipe they were using is made of better, heavier steel than most gun barrels. Considering that it is very heavy because you need a lot of weight to grind up the rocks with a bit, the expense would have been staggering. When they broke into the magma chamber the molten rock would have surged on them, blowing much of the drilling mud out of the hole because of instant steam formation, and it could have possibly hurt someone.

Moreover the elevated temperatures as the magma chamber is approached will create major problems. The lubricant in roller cone bits will have long since been baked, so the bit fails. The diamond table on PDC bits will suffer thermal degradation and the bit will stop drilling. Drilling fluid properties will degrade to the point where the mud fails to function. Overall a rather bad idea.

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